This application claims priority of the German patent application 102 34 757.3-51 which is incorporated by reference herein.
The invention concerns an autofocus module for a microscope-based system. The invention concerns in particular an autofocus module for a microscope-based system having an objective that defines an image beam path which is perpendicular to a surface of a specimen and can be focused thereonto, and having an illumination beam path that encompasses a light source for illumination of the specimen.
German Patent DE 32 19 503 discloses an apparatus for automatic focusing onto specimens to be viewed in optical devices. After reflection at the surface of a specimen, the reflected measurement light beam bundle passes through a pinhole after reflection at a splitter mirror. A portion of the measurement light beam bundle is reflected out by means of a fully mirror-coated surface, and after passing through a slit aperture is directed onto a differential diode. In the focused state, the focus is located between the two diodes. In the event of defocusing, the measurement spot drifts onto one of the two diodes, which are connected to corresponding control means. The control means adjust optical or mechanical means of the microscope in order to bring the measurement spot back between the two diodes and thus reestablish the focus position.
European Patent Application EP-A-0 124 241 discloses a microscope having an automatic focusing device. The microscope encompasses a memory device for saving the data from the objectives that are used in the microscope. Also provided is a control device which monitors and regulates the various microscope functions. Another of the tasks of the control device is to move the focusing stage. A CCD element, which receives an image from the particular selected objective and, together with a computation unit, determines the image sharpness based on optimum contrast, is provided as the image acquisition device. The objective data of the objective presently in use must be taken into account when determining the optimum sharpness. Those data are, as mentioned above, stored in a memory.
German Unexamined Application DE 41 33 788 furthermore discloses a method for autofocusing of microscopes, and an autofocus system for microscopes. The image of a specimen or of a pattern reflected onto the specimen is conveyed to two areas on a detector or to two different detectors; in the focused position, one image is produced in front of one detector, and one image behind the other detector. The image sharpness states on the detectors are converted into electronic signals, whose difference is used to focus the objective. The distances of the image or of the respective pattern from the respective detectors are adjustable. Deliberate offset settings, as well as xe2x80x9cIR offsetxe2x80x9d correction settings, can be implemented.
A problem in the context of automatic focusing in microscopes in the semiconductor industry is that with small focus spots, the location of the focus spot is important. For example, if the focus spot is located on an elevation of the topology, focusing occurs there. If the focus spot is located, for example, in a valley of the topology, focusing occurs onto the valley. It is self-evident that different images are acquired depending on the focus position. This has a negative effect in terms of digital image processing, however, since it can happen that images which do not reproduce the optimum information content are processed.
It is accordingly the object of the present invention to create an autofocus module for a microscope-based system with which optimum and automatic focusing can be achieved regardless of the particular topology of the specimen being examined.
The object is achieved by way of an autofocus module for a microscope-based system which comprises:
an objective that defines an image beam path which is perpendicular to a surface of a specimen and can be focused thereonto, and an illumination beam path that encompasses a light source for illumination of the specimen,
a light source that generates a measurement light bundle for determining at least one focus position;
an optical element for splitting the measurement light bundle in such a way that an eccentrically extending annularly divergent measurement light bundle is created; and for parallelizing a divergent measurement light bundle remitted from the microscope-based system;
a first dichroic beam splitter positioned in the image beam path of the microscope-based system, for coupling the eccentrically extending measurement light bundle eccentrically into the microscope-based system and for directing it onto the surface of the specimen; and
at least one optical means for directing the remitted measurement light beam bundle onto a differential diode.
The use is particularly advantageous because the eccentrically extending measurement beam bundle is shaped into a ring and thus covers a larger area on the specimen. The determination of the focus position is thus averaged over several different topological locations. The focus is thus set regardless of the changing topology in one region of the specimen. In addition, the differential diode comprises a first and a second diode. From the distribution of intensities that are measured on the two diodes, conclusions can be drawn as to the focus position. The motion of the focus spot on the differential diode yields the direction of motion of the surface of the specimen relative to the optimum focus position. A computer or control system is provided so that based on the data from the differential diode, the surface of the specimen can be brought automatically into the optimum focus position.